1. Field of the Invention
The present invention relates to a method of forming a high quality, stable and compact native oxide layer from an aluminum-bearing Group III-V semiconductor material. More specifically, the present invention forms the native oxide layer by a method involving wet thermal oxidation. Importantly, the thickness of the native oxide layer produced by the method is substantially the same as or less than the thickness of the aluminum-bearing Group III-V material layer that converts to the oxide. Further, the present invention forms the native oxide under conditions that discourage the formation of various other oxygen-rich compounds, such as aluminum oxide hydrates and aluminum suboxides, the presence of which compounds cause an expansion of the resultant native oxide layer thickness and are generally deleterious to the electrical and physical properties of the semiconductors.
The present invention is also directed to devices utilizing the native oxide layer thus grown, including electrical and optoelectrical devices such as transistors, capacitors, waveguides and, more especially, lasers.
Finally, the present invention relates to the masking and passivation of semiconductors utilizing the native oxide that forms from the practice of the present invention.
2. Description of the Prior Art
An important trend in semiconductor technology is the use of Group III-V materials for the fabrication of semiconductor devices. While the utilization of silicon (Si) is still prevalent in this area, Group III-V compounds--such as GaAs--have been the subject of much research due to the significant advantages these compounds offer. For example, Group III-V compounds generally exhibit larger band gaps, larger electron mobilities and have the ability to produce light, which properties result in unique electrical and optical characteristics.
Notwithstanding these qualities, Group III-V semiconductor technology has failed to develop at the rate and to the level of silicon-based technology. The primary causative factor to this end has been the inability to produce, on the Group III-V semiconductor, an oxide layer of desired thickness that exhibits the necessary surface state and electrical properties required for practical application. In this regard, the oxide must be able to fulfill, without the disruption and strain caused by over-expansion of the oxide thickness, a variety of functions in a practical and consistent manner. Examples of these functions include: serving as a mask during device fabrication, providing surface passivation, isolating one device from another (dielectric isolation, as opposed to junction isolation), acting as a component in the anatomy of various device structures and providing electrical isolation of multilevel metallization systems. Accordingly, the presence of a high-quality, stable oxide layer having adequate physical properties and proper thickness is essential to the successful development of Group III-V semiconductor technology.
Silicon-based materials, unlike Group III-V semiconductors, readily form a high quality oxide (SiO.sub.2) by such methods as reacting the silicon crystal with water vapor, e.g., in the form of steam. Indeed, the very existence of silicon-based integrated circuit technology is largely due and owing to this ability of silicon to form a high quality silicon oxide. Moreover, this oxide is a native (or natural) oxide, as opposed to a deposited oxide layer. Native oxides are more desirable than deposited oxides in that they are monolithic with the crystal and thus avoid potential mismatching of dielectric characteristics and problems associated with oxide-substrate interface bonding, such as lifting and cracking. Further, deposition processes are on the whole more complicated and costly than are methods of growing a native oxide thus making the latter more attractive for commercial use.
Attempts at producing a quality native oxide layer on Group III-V semiconductors by adapting methods that have been successful for silicon have had disappointing results. These results are usually ascribed to the fact that the behavior of Group III-V materials depends, in large part, on the behavior of the individual Group III-V constituents, which behavior, under given circumstances, may not be compatible with the desired end result. For example, thermal oxidation techniques, which are regarded to be among the simplest of the techniques and which have had tremendous success for silicon, have not worked well for Group III-V materials such as GaAs. This is because gallium (Ga) and arsenic (As) have different oxidation rates, and because the As.sub.2 O.sub.3 and As.sub.2 O.sub.5 that are produced in the normal course of events, are volatile: once formed, they tend to boil off the substrate rather than stabilize on it as part of an oxide layer.
Thus other approaches, which for the most part occur at low temperatures, e.g., room temperature, to avoid the formation of volatile components, to produce a native oxide layer directly from a Group III-V semiconductor surface have evolved. These techniques include the use of ozone, simultaneous O.sub.2 and electron beam exposure, photo-excitation of electron-hole pairs (in GaAs), use of more reactive oxidizers (such as N.sub.2 O), photochemical excitation of the gas-phase molecular species, addition of water to the O.sub.2, excitation of O.sub.2 with a hot filament or a Tesla discharge, plasma excitation of the O.sub.2 and exposure to a high kinetic beam of atomic oxygen. The drawback of these techniques, aside from their overall complexity, which makes them unrealistic for large scale utility, is that although they can increase the rate of formation of the first few monolayers of oxide they are (with the possible exception of plasma oxidation and exposure to a high kinetic beam of atomic oxygen) generally ineffective for rapidly growing layers having a thickness in the range of hundreds to thousands of angstroms, .ANG. (10,000 .ANG.=1 micron, .mu.m). Moreover, these oxidation reactions are often incomplete, the Ga and As not being in their highest formal oxidation state. The resulting oxide is thus usually deficient in Ga or As which deficiencies have adverse effects on oxide quality.
Particular examples of these methods include: U.S. Pat. No. 3,859,178 wherein an oxide is grown on the surface of a GaAs layer by submersing the GaAs layer into an anodization bath of concentrated hydrogen peroxide (H.sub.2 O.sub.2) having a pH of less than 6.
U.S. Pat. No. 4,374,867 describes a method of growing an oxide layer on InGaAs by using a growth chamber that has been evacuated and in which an oxygen plasma has been established. Water vapor is introduced into the chamber to facilitate the growth process.
U.S. Pat. No. 3,890,169 relates a method of forming an oxide on GaAs in an electrolytic fashion using H.sub.2 O.sub.2 as an electrolyte. The oxide thus formed is rendered more stable and more impervious to impurities and dopants normally employed in diffusion processes by being dried in oxygen at 250.degree. C. for 2 hours followed by annealing at 600.degree. C. for 30 minutes.
U.S. Pat. No. 3,914,465 describes a double oxidation technique whereby a native oxide is grown on GaAs by immersion in an aqueous H.sub.2 O.sub.2 solution with a pH of 1.5-3.5, followed by a second oxidation in aqueous H.sub.2 O.sub.2 at a pH of 6-8.
H. Barbe, et al. in Semiconductor Science and Technology, 3, pp. 853-858 (1988) describe the growth of a thin oxide layer on GaAs in methanol having a varying water content, without the application of external voltage. J. P. Contour, et al. in the Japanese Journal of Applied Physics, Vol. 27, No. 2, pp. L167-L169 (Feb. 1988) report on the preparation of a surface oxide on a GaAs substrate by heating the substrate to 250.degree.-350.degree. C. in air. Similarly, in Applied Physics Letters, Vol 26, No. 4, pp. 180-181 (Feb. 15, 1975), the growth of an oxide film on GaAs by thermal oxidation at 350.degree., 450.degree. and 500.degree. C. is described. Applied Physics Letters, Vol. 29, No. 1, pp. 56-58 (Jul. 1, 1976) reports on a one step dry process to form an oxide film on GaAs by plasma oxidation using an oxygen plasma.
Because of the complexity of these techniques and the less-than-desirable results in terms of physicality and thickness obtained, all of which can be related to the difficulties in working with Ga and As, methods of oxide formation have been developed which involve overlaying or implanting on a Group III-V surface a material that can oxidize more readily. Aluminum (Al) and aluminum-bearing compounds are examples of such materials. These particular materials are particularly adaptable in that aluminum is a Group III element and is known to oxidize more easily than the other elements normally found in Group III-V semiconductors.
Examples of oxidation methods which exploit the presence of aluminum or aluminum-bearing compounds include U.S. Pat. No. 4,144,634 which first deposits a thin layer of Al by, e.g., evaporation, over a GaAs substrate. The Al overlay is then oxidized by plasma oxidation. Y. Gao, et al. report in the Journal of Applied Physics, 87, (11), pp. 7148-7151 (Jun. 1, 1990) a cryogenic technique whereby molecular oxygen is first overlaid on a GaAs surface; deposition of Al follows. The Al reacts to form an oxide layer until the oxygen is depleted.
C. W. Wilmsen, et al. in Thin Solid Films, 51, pp. 93-98 (1978) report a method whereby a metal, such as Al, is implanted into a Group III-V substrate; oxidation then occurs by thermal or anodic means. M. Hirose, et al. relate in Physica Status Solidi, (a) 45, pp. K175-K177 (1978) an oxidation process for GaAs in which oxygen gas, admitted close to the substrate surface, is reacted with Al molecular beams to form Al.sub.2 O.sub.3. Finally, U.S. Pat. Nos. 4,216,036 and 4,291,327, and European Patent Application 0 008 898 describe the fabrication of oxides by the thermal oxidation of an AlAs or AlGaAs layer which has been epitaxially grown on GaAs. The oxidation occurs in a flowing gas mixture of 80% O.sub.2 and 20% N.sub.2, and can occur in the presence of water vapor in order to permit the use of lower temperatures, e.g., 70.degree.-130.degree. C.; the oxides produced by this method are, however, believed to be aluminum arsenic oxide and/or hydrated aluminum oxides. These types of oxygen-rich aluminum compounds do not have the requisite physical characteristics that are necessary for semiconductor application; moreover, their presence in any modest amounts is deleterious to semiconductor structure. In addition to this, and integrally related to the presence of hydrates, is the expansion of thickness in the final oxide layer, which is consistently 80% thicker than the thickness of the original AlAs epilayer In terms of real application and device construction, this magnitude of layer expansion is wholly impractical in that it distorts and strains the device architecture to unacceptable levels and puts inter-dependent dimensions and geometry out of kilter. These shortfalls are especially harmful when the semiconductor device is an optoelectrical device such as a laser, the optical output efficiency and lifespan of which is highly dependent on proper crystal dimensioning and geometry as the various layers are developed over the course of device fabrication.
In brief, prior art methods which rely on the presence of materials such as aluminum, are either too complex for large scale use or result in oxides that contain significant amounts of hydrates and/or have thicknesses which are over-expanded. The oxides produced by these methods also have less-than-desirable physical and electrical characteristics, in that they have poor electrical properties, e.g., significant leakage, and the overall quality of their physical state is not good. As to the latter, oxides formed by these known methods exhibit non-uniformities in density and continuity, and also lack suitable stability, which results in lifting, cracking and out-diffusion; devices fabricated with oxides grown by these methods show a strong tendency to degrade in unacceptably short periods of time under normal conditions of use and atmospheric exposure. These undesirable end results and deleterious effects thus preclude the use of these methods in large scale practical application as required for commercial devices.
Thus the semiconductor art, although producing a variety of methods to form oxides on Group III-V semiconductor materials, recognizes a continuing need for a method of growing an improved, high-quality native oxide on aluminum-bearing Group III-V semiconductor materials, particularly a native oxide whose thickness is substantially the same as or less than the thickness of the semiconductor material from which it forms. Moreover, it is desirable that the method be simple, cost effective and produce the native oxide consistently in a controlled and repeatable manner.